1. Field of the Invention
The invention relates to an apparatus and a method for controlling exhaust emission of an internal combustion engine.
2. Description of the Related Art
There are known technologies related to an exhaust emission control apparatus in which a NOx catalyst (NOx retainer agent) capable of absorbing NOx from incoming exhaust gas is provided, and a SOx catalyst (sulfur component retainer agent) capable of absorbing SOx contained in incoming exhaust gas is disposed upstream of the NOx catalyst. In such an exhaust emission control apparatus, the provision of the SOx catalyst upstream of the NOx catalyst prevents the sulfur poisoning of the NOx catalyst caused by inflow of SOx into the NOx catalyst.
However, the SOx catalyst used in the aforementioned related-art exhaust emission control apparatus is not capable of infinitely taking up and retaining SOx from incoming exhaust gas, but has a limit to the amount of SOx retainable. Generally, as the amount of SOx retained by the SOx catalyst increases, it becomes more difficult for the SOx catalyst to capture and retain SOx contained in incoming exhaust gas; therefore, there is a tendency toward an increased amount of SOx contained in exhaust gas flowing out of the SOx retainer agent. Such an increase in the amount of SOx contained in exhaust gas flowing out of the SOx retainer agent will result in the sulfur poisoning of the NOx retainer agent in the aforementioned related-art exhaust emission control apparatus. Hence, the amount of SOx contained in exhaust gas flowing out of the SOx retainer agent needs to be small even if the amount of SOx retained in the SOx retainer agent becomes great.